A charged particle is an elementary particle or a macroparticle that contains an excess of positive or negative charge. A collection of charged particles is referred to as a “particle beam.” The motion of a charged particle is largely determined by interaction with electromagnetic forces. A charged particle accelerator does work, and thus imparts kinetic energy to a charged particle by application of an electric field.
Kinetic energy is imparted to charged particles via electromagnetic forces. For example, a power supply may generate a voltage difference between a pair of metal plates by subtracting negative charge from the first plate and moving it to the second plate. A charged particle that moves between the metal plates is accelerated by forces associated with the electric field developed between the two plates ({right arrow over (F)}=q{right arrow over (E)}). Magnetic forces, acting in a direction transverse (i.e., perpendicular) to the velocity of the charged particle, keep a charged particle within a specific cross-sectional area and curve the path of the charge particle. Accordingly, the magnetic forces act as “confinement forces.”
Particle accelerators are used in a wide variety of applications, including, but not limited to, generation of X-rays, sterilization of food products, modification of properties of materials, production of isotopes, manufacture of semiconductors, and medical and scientific research applications.
Cyclotrons and microtrons are examples of prior art circular particle accelerators. A cyclotron is an apparatus that accelerates charged particles by using a high frequency alternating voltage across a magnetic field to spiral a charged particle in a generally circular path. More specifically, a cyclotron is generally comprised of two empty, semicircular D-shaped chambers, known as “dees.” The two chambers are arranged relative to each other to define a narrow, empty slot therebetween. The dees are placed in a vacuum chamber between the poles of an electromagnet. A high frequency AC voltage is supplied to the dees to generate an electric field. A charged particle source injects charged particles into the vacuum chamber, wherein the charged particles are accelerated in the gap between the dees.
The cyclotron has several drawbacks. In this regard, a cyclotron has a magnetic field of constant magnitude and a constant radiofrequency AC voltage. The beam energy is limited by relativistic effects that destroy synchronization between particle orbits and radiofrequency fields. Accordingly, the cyclotron is not suitable for accelerating all types of ions.
The microtron combines linear accelerator technology with circular accelerator particle dynamics, and can produce a continuous beam of high-energy electrons with an average current of about 100 μA. One common type of microtron is known as a racetrack microtron. Electrons are accelerated in a short linear accelerator section. Magnets at each end of the linear accelerator confine the electrons to recirculate the beam through the linear accelerator. In this regard, the magnets produce uniform magnetic fields that cause the electrons to orbit half-circles that return the electrons to the linear accelerator. The size of the orbit increases as electron energy increase.
Among the drawbacks of microtrons are problems with beam steering and beam breakup instabilities. With regard to beam steering, the uniform magnetic field has horizontal focusing but no vertical focusing. Beam breakup instability is severe in the microtron because the current of all beams is concentrated in the high charge resonant cavities of the linear accelerator. The beam breakup instability limits the average current to less that 1 mA. Microtrons also have the drawback that they are limited to use with electrons.
The present invention addresses drawbacks of the prior art, and provides a novel method and apparatus for accelerating charged particles.